In a magnetic disk drive, a head (magnetic head) needs to be positioned at a target position on a disk (magnetic disk). Servo data is used for the head positioning. The servo data is, for example, discretely recorded at predetermined intervals in tracks (servo tracks) concentrically arranged on the disk. Areas in which the servo data is recorded are referred to as servo areas or servo frames. The servo data may also be referred to as servo frames.
The area between adjacent servo areas (servo frames) in a circumferential direction of the disk is used as a data area (data frame). The length of the data frame depends on the length of the servo frame. Thus, the possibility of reducing the servo frame length enables an increase in data frame length, that is, in the linear recording density of the disk. Hence, as an approach to increasing the linear recording density, various techniques have been proposed which optimize the servo frame length by varying a recording frequency for servo data, for example, with each head and with each zone.
In general, the servo data (servo frame) includes servo preamble data. The servo preamble data is used for generation of a servo clock signal using a phase-locked loop (PLL) and for phase acquisition (acquisition of the phase of a servo clock frequency). The servo clock signal is used to read the servo data.
The length of the servo preamble data (that is, servo preamble length) is set to a value with an appropriate margin so as to allow the phase of the servo clock frequency (the frequency of the servo clock signal) to be reliably acquired.